An optical communication system needs an optical attenuator to control the amount of light passing therethrough. The optical attenuator is configured to provide a polarizer and an analyzer at the front and rear (the incoming side and the outgoing side) of the optical axis of a Faraday rotational angle variable device. The Faraday rotational angle variable device to be integrated applies external magnetic field to a garnet single crystal having a Faraday effect in two or more directions, and by varying these synthesized magnetic field, it controls the Faraday rotational angle of a light beam passing through the garnet single crystal. The optical attenuator controls the amount of attenuating light, by controlling the Faraday rotational angle.
On the optical attenuator, for the polarizer and the analyzer, the use of a compound polarizing prism may be allowed as a principle, however, generally, it is more practical to use a wedge-shaped birefringent crystal on a fiber combination type device to make the device of a polarization non-dependence, because if the compound polarizing prism is used, the amount of incident light will be reduced nearly to half by the polarizer. FIG. 1 shows an example thereof.
As shown in FIG. 1A, between an input fiber 11 having a collimate lens 10 and an output fiber 13 having a collimate lens 12, a polarizer 14 comprising a wedge type birefringent crystal (for instance, a rutile crystal), a Faraday rotational angle variable device 15, and an analyzer 16 comprising a wedge type birefringent crystal (for instance, a rutile crystal) are located on the optical axis, in that order. As shown in FIG. 1B, the Faraday rotational angle variable device 15 comprises a combination of a garnet single crystal 17, a pair of permanent magnets 18 to apply magnetic field to the garnet single crystal from two different ways, 90 degrees apart from each other, and an electromagnet 19. The permanent magnets 18 are angular shaped and magnetized in the thickness direction, and generate a magnetic field of a parallel direction to the optical axis. The electromagnet 19 to generate a magnetic field in the vertical direction to the optical axis is configured so that it can vary the intensity of a magnetic field by the current to be passed through its coils. Therefore, the magnetizing direction of the garnet single crystal 17 will be varied, depending on the direction of the combined magnetic field of the fixed magnetic field by the permanent magnet 18 and the variable magnetic field by the electromagnet 19, and a Faraday rotational angle will be changed depending on the change made in the magnetizing direction.
Herein, as shown in FIG. 2, the polarizer 14 and the analyzer 16 have so far been located so that the optical axes of both of these birefringent crystals can form an angle of −52.5 degrees or +52.5 degrees from a horizontal surface, viewed from the optical axis direction (in other words, an angle formed by both axes is 105 degrees), and the magnetic field by the electromagnet is directed to a horizontal direction. The light coming out of the input fiber 11 will be turned into a collimated beam by the collimate lens 10, and will be further separated, by the polarizer 14, into a normal light o that is parallel to its optical axis and an abnormal light e that is vertical to its optical axis. (Therefore, the polarization directions of the normal light o and the abnormal light e intersect each other at a right angle.) And, when the individual light beams pass through the Faraday rotational angle variable device, the polarization directions will turn, depending on the size of magnetization in the direction parallel to the optical axis, and by the analyzer 16, each of these light beams will be separated into a normal light o1 and an abnormal light e1, or a normal light o2 and an abnormal light e2. The normal light o1 and the abnormal light e2 coming out of the analyzer 16 are parallel to each other, and will be combined to the output fiber 13 by the collimate lens 12 (represented by a solid line), however, the abnormal light e1 and the normal light o2 coming out of the analyzer 16 will not be combined to the output fiber 13, even if they pass through the collimate lens 12, because these light beams will spread out not in parallel lines (represented by a dotted line).
When the number of magnetic field to be applied by the electromagnet 19 is zero, the magnetization direction of the garnet single crystal is parallel to the optical axis, and the Faraday rotational angle will become maximum. As the normal light o coming out of the polarizer 14 will be sent out from the analyzer 16 as the normal light o2, and the abnormal light e coming out of the polarizer 14 will be sent out from the analyzer 16 as the abnormal light e2, both of the light beams are parallel and will be combined to the output fiber 13 by the collimate lens 12. On the contrary, when the magnetic field applied by the electromagnet 19 is large enough, the Faraday rotational angle will become minimum, and as the normal light o coming out of the polarizer 14 will come out of the analyzer 16 as the abnormal light e1, and the abnormal light e coming out of the polarizer 14 will come out of the analyzer as the normal light o2, these light beams will be difficult to be combined to the output fiber 13, even if they pass through the collimate lens 12. As described above, depending on the intensity of the magnetic field to be applied by the electromagnet 19, the magnetization of the garnet single crystal 17 turns, with the Faraday rotational angle changing in a certain range of angles, thereby changing the amount of light beams to be combined to the output fiber 13, thereby enabling the device to perform a function as an optical attenuator.
Here, the reason why the optical axes of the birefringent crystals that make up the polarizer and the analyzer are set at −52.5 degrees or +52.5 degrees from a horizontal surface, respectively, and an angle between these axes is set at 105 degrees is because of the resultant merits, including that the same shaped wedge type birefringent crystal can be used both for the polarizer and the analyzer, so that satisfactory productivity can be expected, and as described below, that the amperage to be supplied to the electromagnet can be lowered, and a large amount of optical attenuation can be obtained.
In view of circumstances of the power to be added to the electromagnet, the electromagnet is set at an angle of 90 degrees or more, when the magnetization is facing toward the light beam direction, and allow the angle to change within the angle range less than 90 degrees. For instance, when the magnetization faces toward the light beam direction, the Faraday rotational angle will become 96 degrees, and then, the Faraday rotational angle is reduced to 15 degrees by applying a magnetic field of the electromagnet. In this case, if the angle between the optical axes of both of the birefringent crystals, which function as the polarizer and the analyzer, are set at 105 degrees, a maximum amount of optical attenuation can be obtained when the Faraday rotational angle is 15 degrees.
However, optical attenuators of the conventional configuration have a problem that the range, in which the amount of optical attenuation can be changed, in other words, the dynamic range is too small. Also, another problem is found that the temperature-dependence of the dynamic range is so large that the lower temperature drops, the smaller the dynamic range becomes. For instance, the dynamic range at the room temperature is only about 25 dB at the largest, and when the temperature drops to −10° C. or less, the dynamic range will drop to 20 dB or less.
It is therefore an object of the present invention to provide an optical attenuator that is able to enlarge the range, in which the amount of optical attenuation can be changed, in other words, the dynamic range, and further able to reduce the temperature-dependence of the dynamic range.